The present invention relates generally to methods and devices for extending the operating life of fixed bed reactors. More specifically, the present invention relates to a bypass device that partitions a single fixed bed of a fixed bed reactor into a first top bed and a second virtual bed. The bypass device further bypasses a reactor feedstock around the first top bed as the first top bed fouls and distributes the bypassed feedstock flow to the top surface of the second virtual bed. The fixed bed reactor may have one or more than one fixed beds. The invention also relates to fixed bed reactors that use the inventive bypass device and to methods for extending the operating life of fixed bed reactors.
In the operation of fixed bed reactors, the top of the fixed bed often becomes fouled or plugged by the deposition of fouling materials (also referred to as particulates, particulate impurities, or foulants) contained in the fluid feedstocks flowing into the fixed bed. Examples of fouling materials include organometallic compounds, polymeric materials, carbonaceous materials, organic particulates and inorganic particulates. The plugging of the fixed bed results in increased pressure drop that may necessitate shutdowns, throughput reduction, and time consuming repairs and maintenance.
To overcome this problem, many methods have been devised that require equipping each reactor with more than one fixed bed and bypassing a fouled fixed bed. Examples of such methods are described in U.S. Pat. Nos. 3,509,043 and 4,313,908. One shortcoming of such methods is that they require an auxiliary bypassable fixed bed. Thus, the above methods do not readily apply to single fixed bed reactors.
Other methods involve the use of trash baskets. For example, U.S. Pat. Nos. 3,992,282 and 3,888,633, describe a fixed catalyst bed reactor having a hollow trash basket made from a screen mesh material that extends into the fixed catalyst bed. Particulate impurities are removed from a fluid stream flowing into the fixed catalyst bed by the hollow trash basket.
While the trash baskets remove some fouling materials contained in the fluid feedstocks, they generally have only a small effect in minimizing pressure drop buildup due to fouling. This is partially because fouling materials plug the trash basket walls within a short period of time. Thus, the flow passage of the fluid feedstocks is occluded and the pressure drop begins to rise, though at a somewhat slower rate than if the trash baskets were not used. Generally, it is desirable to keep fixed bed reactors on stream without significant pressure drop buildup for a long time, very often for several years. Thus, the methods involving trash baskets do not provide adequate protection against pressure drop buildup, and other methods are needed to further extend the operating life of fixed bed reactors. Other problems are associated with existing methods for extending the operating life of fixed bed reactors.
One aspect of the present invention relates to a reactor for treating a feedstock flowing therethrough. The reactor comprises at least one fixed bed for treating the feedstock and at least one bypass device. The bypass device comprises a cage positioned within the fixed bed. The cage has a top wall, side walls and a substantially open or wholly open bottom end. The cage partitions the fixed bed into a first top bed and a second virtual bed. The bypass device also comprises a bypass tube that is in fluid communication with the cage. The bypass tube protrudes from the cage above the top surface of the fixed bed for bypassing an increasing amount of the feedstock around the first top bed as it fouls. The bypass flow is directed through the bypass tube into the cage and out from the cage through its open bottom end on the top surface of the second virtual bed.
Another aspect of the present invention relates to a method for extending the operating life of a fixed bed reactor. The method comprises providing a fixed bed having at least one fixed bed, partitioning the fixed bed into a first top bed and a second virtual bed, introducing a feedstock into the fixed bed, and bypassing an increasing amount of the feedstock to the second virtual bed as the first top bed fouls. The present invention bypass device promotes the deposition of any foulants contained in the bypass flow on the top surface of the second virtual bed rather than into the interstices of the second virtual bed.
Yet another aspect of the present invention relates to a bypass device suitable for use in connection with a fixed bed reactor. The bypass device may include a substantially open or wholly open bottom end to maximize the top surface area of the second virtual bed where any bypassed foulants can deposit.
It has been unexpectedly discovered that the use of one or more of the inventive bypass devices on fixed bed reactors reduces significantly the pressure drop buildup of the fixed bed reactors. Methods, and systems that employ bypass devices having cages with fully open bottom ends are preferred, however, mechanical or other reactor specific constraints may prevent the bottom end of a cage to be fully open. Also, wire meshes or grids that are nearly fully or substantially open to the flow may also be used at the bottom of the cage as long as they do not obstruct the top surface area of the virtual bed to any significant extent.
A single larger cage or several smaller cages may be used to create a second virtual bed and maximize the top surface area of this virtual bed. Also, some of the cages may have multiple bypass tubes or no tubes at all. Many other variations or embodiments of the present invention, may be used to create a second virtual bed and maximize its top surface area that is available for foulant deposition. These and other embodiments of the present invention will become better understood with reference to the following detailed description considered in conjunction with the accompanying drawings described below.